as you may know i have never used hpa and therefore don't know much about it. my question is: how many shots can i get out of..say a 68-4500 tank. i know this can change from gun to gun..but lets say its on an average gun (epiphany, eos, new slg, etek ) with stock everything. about how many shots can i expect before in need a refill?

i was wondering why the "ultra" tanks cost more, even though they appear to be physically smaller?
and what does it mean when the tank is "fixed" does that mean that the output pressure cannot be changed at all?
btw, is pure energy a good brand? otherwise what kind should i get??

Those "ultra" tanks have an additional few layers of black epoxy on top to protect the actual carbon fiber/epoxy weave. It's the best solution if you intend to keep a tank for a long time and keep rehydro'ing it.

Fixed regulators have a set output. It is possible with PureEnergy, Ninja Paintball (Ninja includes everything stock, best choice for this) and Crossfire tanks to purchase new internals and swap between high, mid, and low pressure outputs. Those three brands are currently the ones I own, and love. If you choose another brand, make sure it isn't the Myth. They ping, have piston stick, and are just generally horrible regulators.